Electrical and thermal properties of carbon nanotube bulk materials: Experimental studies for the 328-958 K temperature range

We report on electrical and thermal properties in the temperature range from 328 to 958 K of multiwall carbon nanotube (MWNT) bulk materials that were consolidated by spark plasma sintering. The rather dense MWNT bulk materials show exclusively nonmetallic temperature dependence of electrical conductivity from 328 to 958 K, owing to the absence of metallic conduction mechanism in such a highly disordered system. The conductivity exhibited extremely weak temperature dependence with only 35% increase of room-temperature conductivity at 958 K, which was explained by a heterogeneous model considering both fluctuation-assisted tunneling between nanotubes or shells of MWNT and variable-range hopping between graphite microphases that were observed to be dispersed in MWNT bulk materials. The results suggest that fluctuation-assisted tunneling governed this weak conductivity-temperature dependence. Metallic diffusion behavior was observed from 328 to 958 K, and it indicates that phonon drag contributed little to the thermoelectric power of MWNT bulk materials. By contrast, we further show that the increase in sample dimensionality from individual MWNT to bulk materials tends to increase the metallic temperature dependence of electrical conductivity and remarkably decrease the magnitude of thermal conductivity. The geometric shift from graphene sheet to tubular nanotube for carbon-related bulk materials changes the conduction from the combination of n and p types to absolute p type.